Not applicable.
Not applicable.
1. Field of the Invention
The invention relates to a reticle whose transparent support material consists of an optically uniaxial crystal.
2. Discussion of Relevant Art
A reticle for 100-200 nm lithography whose substrate is to consist of MgF2, or equally well of CaF2 or diverse other fluorides, is described in German Patent DE 34 17 888 A (British Patent document 21 39 781). Manifestly only their transparency in the given wavelength region is considered as the selection criterion. Nothing is said concerning the crystal structure, birefringence, thermal expansion, and polarization.
In the Technical Publication TP 58401,157 nm lithography with transparent optical elements, of MgF2 among others, is described without being specified in more detail.
MgF2 is a typical optically uniaxial crystal.
A radially polarization-rotating optical arrangement, and a microlithography projection illumination equipment therewith, is described in German Patent DE 195 35 392 A.
Lithography with the excimer laser wavelength of 157 nm can no longer fall back on the proven quartz glass as reticle support material, since quartz glass is opaque in the given spectral region. Isotropic CaF2 has a drastically higher linear thermal expansion coefficient of 18.9xc2x710xe2x88x926/xc2x0K, as against 0.5xc2x710xe2x88x926/xc2x0K for quartz glass.
The already proposed MgF2 (magnesium fluoride) has a markedly smaller thermal expansion. Based on the crystal structure, however, not only is this crystal optically uniaxially birefringent, but also the thermal expansion is anisotropic.
Pellicles are thin diaphragms for the protection of the mask structure on the reticle. Besides organic foils, SiO2 pellicles are also known. Japanese Laid-Open Patent Publication JP-A-481756 describes a pellicle in which a fluoropolymer is coated on both sides with CaF2.
The invention has as its object to provide a reticle, which is suitable for wavelengths in the 100-200 nm region and which is improved as regards its thermal and optical properties. The same holds for a pellicle according to the invention.
The object is attained by a reticle according to the invention with support material of transparent, optically uniaxial crystal, in which the principal axis (A) of the crystal is substantially perpendicular to the surface of the reticle. Advantageously, the support material is MgF2According to the invention, the axial direction of the crystalline support is oriented such that the thermal expansion within the reticle surface is homogeneous. Ideally, this is obtained with an exactly perpendicular orientation of the crystal axis. However, deviations arising from manufacturing technology, for example, can be tolerated to the extent that the resulting increasingly unequal thermal expansion can be tolerated. The tolerance of 5xc2x0 represents a measure above which the embodiment would be little appropriate.
MgF2 is the preferred optically uniaxial crystal.
The disturbance of the microlithographic imaging by thermal expansion can be additionally reduced by means of a cooling device according to an advantageous feature of the invention.
An optimization of the optical properties also results according to an advantageous feature of the invention from bonding the thus constituted reticle into an illumination equipment that provides radially polarized light. The refraction at the crystal is thereby no longer dependent on direction. For the tolerance against deviations of the optical axis from the crystal axis, the above-mentioned correspondingly holds, and hence it is advantageous to include in such illumination equipment the features described above.
An illuminating device is thus provided with a reticle that is transparent at light wavelengths of 100-200 nm, in which anisotropies of thermal expansion and of refraction play no part, and the absolute amount of the thermal expansion is halved in contrast to CaF2 (9.4xc2x710xe2x88x926/xc2x0K) as the most prevalent comparison material.
A further advantageous embodiment of the reticle according to the invention is provided by suitable cooling.
The constitution of a pellicle according to the invention consists of a fluoride crystal, preferably CaF2, or BaF2 or MgF2.